1. Field of the Invention
The present invention relates to a gold alloy wire for wedge bonding and a use of said gold alloy wire in wedge bonding.
2. Description of the Related Art
A wire bonding method is known to connect electrodes of an IC chip with outer terminals and the like through wires. The wire bonding method includes among others a ultrasonic and thermal pressing bonding and a ultrasonic bonding, classified by the method of bonding a wire to an electrode of an IC chip.
A typical ultrasonic and thermal pressing bonding is a nail head bonding method. The nail head bonding method is illustrated in FIGS. 1A to 1D.
Referring to FIG. 1A, a wire 2 is arranged through a capillary 1, a torch 3 faces a tip of the wire 2, and an electric discharge is generated between the torch 3 and the wire 2 to heat and melt the tip of the wire 2 to thus form a ball 4.
Referring to FIG. 1B, the capillary 1 is lowered and the ball 4 is pressed to an A1 electrode 5 of an IC chip 6. At this time, ultrasonic waves are applied to the ball 4 through the capillary 1 and the IC chip 6 is heated by a heater block, so that the ball 4 is thermally pressed and bonded to the electrode 5 and becomes a bonded ball 4'.
Referring to FIG. 1C, the capillary 1 is moved through a certain route (wire 2') toward above an outer lead 8 and is then lowered to the outer lead 8. At this time, ultrasonic waves are applied to the wire 2 through the capillary 1 and the outer lead is heated by a heater block, so that side sections of the wire are thermally pressed and bonded to the lead 8.
Referring to FIG. 1D, a damper 7 is lifted while clamping the wire 2 so that the wire 2 is cut, and thus the wire bonding is finished.
On the other hand, a typical ultrasonic bonding method is a wedge bonding method using a wedge bonding tool.
Referring to FIG. 2A, a wire 12 is arranged through a lower end of a wedge bonding tool 11 and an A1 electrode 15 of an IC chip 16 is moved below the wedge bonding tool 11.
Referring to FIG. 2B, the wedge bonding tool 11 is then lowered and ultrasonic waves are applied at room temperature, i.e. without heating, to the wedge bonding tool 11 to the pressed wire 12, to bond the wire 12 to the A1 electrode 15 of the IC chip 16.
Referring to FIG. 2C, a clamper 12 releases the wire and the wedge bonding tool 11 is moved through a certain route (wire 12') to above an outer lead 18 and then lowered onto the outer lead 18. At this time, ultrasonic waves are again applied to the wire 12 at room temperature through the wedge bonding tool 11, to bond the wire 12 to the outer lead 18.
Referring to FIG. 2D, a clamper 17 is lifted while clamping the wire 12 so that the wire 12 is cut, and thus the wire bonding is finished.
The above nail head bonding method is preferred since it is excellent in productivity, but it is used in combination with a gold alloy wire since it involves heat and it is not suitable for an aluminum alloy wire which is easily oxidized when heated.
Further, as shown in FIG. 3A, the ball 4' has a diameter L.sub.1 three to four times larger than the diameter D of the wire 2', which prevents fine wiring.
The wedge bonding is used using an aluminum alloy wire since it can be done at room temperature although its productivity is not high. The wedge bonding however has an advantage that the width L.sub.2 of the deformed wire 14' can be only 1.5 to 2.5 times the diameter of the wire 12', as shown in FIG. 3B.
However, the gold alloy wire is the most preferable material as a wiring material for providing reliability against corrosion in a semiconductor device, since the gold alloy wire is more excellent in resistance against corrosion than other wire materials.
On the one hand, dense wiring is demanded in recent semiconductor devices. To attain this, it is required that the width of the bonded portion of the gold alloy wire with an IC chip electrode is reduced (the width here means the length in the direction perpendicular to the length of the wire).
In this respect, reduction of the width or diameter of the bonded ball has been attempted in nail head bonding using a gold alloy wire, but it has a certain limitation.
The inventors, considering the above problems and demands, have tried wedge bonding to an IC chip electrode using gold alloy wires which have been used or proposed for nail head bonding. It was revealed that although the width of the bonded portion of the wire may be reduced in comparison with the nail head bonding, the bond strength at the bonded portion of the wire at high temperatures (hereinafter called "high temperature bond strength") which is necessary in practical semiconductor devices during operation thereof, was low and the reliability of semiconductor devices was lowered.
Therefore, the object of the present invention is to provide a gold alloy wire, which can be used in wedge bonding to IC chip electrodes to reduce the width of the bonded portion of the wiring and is thus adapted to high density wiring, and which has an improved high temperature bond strength and thus allows an increased reliability of semiconductor devices.